Processes such as the treatment of an exhaust gas of an internal combustion engine, chemical processes in the context of a Fischer-Tropsch synthesis (carbon monoxide reacts with hydrogen to form hydrocarbon compounds), methanation (carbon dioxide or carbon monoxide reacts with hydrogen to form methane), a Sabatier process (carbon dioxide and hydrogen react to form methane), any exothermic, heterogeneously catalyzed gas phase reaction (that is to say for any exothermic conversion of gases on, for example, solid or liquid catalysts) use a gas mixture that passes across a catalyst. Inter alia, it has been the case until now that pellet catalysts have been used in these processes. The use of pellet catalysts ensures (albeit only to a small extent) a dissipation of the heat that arises during the exothermic reactions. The form of catalysts however entails high installation costs, wherein at the same time, the throughput through the catalytic converter is limited because, for adequate dissipation of heat, it has only been possible to use pipes with small diameters (for pellet catalysts with small cross-sectional area).